Waterbed mattress bags, unless abused, are expected to remain leak-proof for several years. Manufacturers' warranties for three and five year periods are not uncommon. In recent years, bags with interior baffle structures have become popular. One such commercially successful bag is shown and described in said Everard/Mollura patent. That bag has a series of baffles, each a bladder comprising a pair of juxtaposed generally circular plastic "pancakes" peripherally attached together and respectively attached at their central regions to the top and bottom bag layers. The attachment between the bag layers and the respective "pancakes" takes place at a generally circular weld. As the bag is filled with water, the "pancakes" separate and assume first, a biconical configuration, and then a configuration that approaches a fluted cylinder. If the bag is stressed, as by excessive filling or by dynamic loading due to body movement, the baffles assume the straight cylindrical form as they forcefully restrain the upper bag layer. The weld is stressed and a point of failure may develop.
Several solutions have been proposed. One solution is to utilize an interior ring die in the welding process shown and described in U.S. Pat. No. 4,167,432, issued Sept. 11, 1979 to Carlos A. Mollura, entitled PROCESS OF MAKING A WATERBED MATTRESS. The improvement is significant; yet, the danger of failure at the weld is still present even if largely reduced.
Another solution is to utilize oversized pancake bladders that provide a much increased range of separation at the center such that the bladders are never axially stressed whatever may be the static or dynamic load on the bag. The problem, however, is that in order to assemble the bladders as by an assembly jig, the center-to-center spacing of the pancake bladders must be increased. Increasing the center-to-center distance between the pancake bladders correspondingly increases the free spaces within the mattress bag with consequent decrease in wave damping.
The primary object of the present invention is to provide a new pancake bladder structure which is never axially stressed, all without compromising the wave damping characteristics of the mattress. Another object of the present invention is to provide a new pancake bladder structure of this kind which can conveniently be assembled by a simple jig structure.